1. Field of the Invention
The present invention relates to electromagnetic wave-absorption multilayer substrates, and more particularly relates to an electromagnetic wave-absorption multilayer substrate preferably used for an electronic circuit incorporated in an electronic apparatus, such as a mobile phone or an audio visual (AV) apparatus, which is necessary to have anti-noise measurements.
2. Description of the Related Art
In a high-frequency electronic apparatus, such as a mobile phone or a wireless apparatus, in order to suppress leakage of a high-frequency noise generated from an electronic circuit, such as a flexible wiring board or an integrated circuit (IC), and/or in order to prevent a high-frequency noise leaked from an exterior circuit, which is provided outside the high-frequency electronic apparatus, from entering an electronic circuit incorporated therein, an electromagnetic wave-absorption multilayer substrate is generally adhered to the electronic circuit.
As shown in FIG. 17, a related electromagnetic wave-absorption multilayer substrate 101 is formed by fixing two insulating substrates 105A and 105B on a front surface 102a and a rear surface 102b, respectively, of an electromagnetic wave-absorption sheet 102. As the insulating substrates 105A and 105B, pre-impregnated sheet material (hereinafter referred to as “prepreg(s)”) have been frequently used in order to obtain superior adhesion to the electromagnetic wave-absorption sheet 102 as well as in order to improve the insulating properties and the protection properties. As the electromagnetic wave-absorption sheet 102, as shown in FIG. 18, a sheet-shaped part has been used which includes a soft magnetic alloy powder 103 and a binding agent 104. As this soft magnetic alloy, a high-permeability material, such as Sendust, permalloy, Ni—Zn ferrite, or Li-ferrite, has been used in many cases; however, in recent year, a Fe-based metal glass alloy having superior magnetic properties has been used.
When a metal glass alloy is used for the electromagnetic wave-absorption sheet 102, after the metal glass alloy is formed into flat particles which are used as the soft magnetic alloy powder 103 and is then mixed with the binding agent 104, such as a silicone resin or poly(vinyl chloride), the mixture thus obtained is formed into a sheet by a doctor blade method, followed by thermal pressure application, so that the electromagnetic wave-absorption sheet 102 is formed. The electromagnetic wave-absorption sheet 102 composed of a metal glass alloy in the form of flat particles can be formed to have a small thickness by the thermal pressure application, and the density of the metal glass alloy can also be increased; hence, compared to an electromagnetic wave-absorption sheet (not shown in the figure) formed of a different type of soft magnetic alloy powder, an electromagnetic wave suppressing effect can be improved.